What if you were presented with an isoceles triangle and told that its baseangles measure
and
? What could you conclude about
x
and
y
? After completing this Concept, you'll be able to apply important properties about isoceles triangles to help you solve problems like this one.

Guidance

An isosceles triangle is a triangle that has
at least
two congruent sides. The congruent sides of the isosceles triangle are called the
legs
. The other side is called the
base
. The angles between the base and the legs are called
base angles
. The angle made by the two legs is called the
vertex angle
. One of the important properties of isosceles triangles is that their base angles are always congruent. This is called the
Base Angles Theorem.

For
, if
, then
.

Another important property of isosceles triangles is that the angle bisector of the vertex angle is also the perpendicular bisector of the base. This is called the
Isosceles Triangle Theorem
. (
Note this is ONLY true of the vertex angle.
) The converses of the Base Angles Theorem and the Isosceles Triangle Theorem are both true as well.

Base Angles Theorem Converse:
If two angles in a triangle are congruent, then the sides opposite those angles are also congruent. So for
, if
, then
.

Isosceles Triangle Theorem Converse:
The perpendicular bisector of the base of an isosceles triangle is also the angle bisector of the vertex angle. So for isosceles
, if
and
, then
.

Example A

Which two angles are congruent?

This is an isosceles triangle. The congruent angles are opposite the congruent sides. From the arrows we see that
.

Example B

If an isosceles triangle has base angles with measures of
, what is the measure of the vertex angle?

Draw a picture and set up an equation to solve for the vertex angle,
. Remember that the three angles in a triangle always add up to
.

Example C

If an isosceles triangle has a vertex angle with a measure of
, what is the measure of each base angle?

Draw a picture and set up and equation to solve for the base angles,
.

Guided Practice

3. True or false: Base angles of an isosceles triangle can be right angles.

Answers:

1. The two angles are equal, so set them equal to each other and solve for
.

Substitute
; the base angles are
, or
. The vertex angle is
.

2. The two sides are equal, so set them equal to each other and solve for
.

3. This statement is false. Because the base angles of an isosceles triangle are congruent, if one base angle is a right angle then both base angles must be right angles. It is impossible to have a triangle with two right (
) angles. The Triangle Sum Theorem states that the sum of the three angles in a triangle is
. If two of the angles in a triangle are right angles, then the third angle must be
and the shape is no longer a triangle.

Practice

Find the measures of
and/or
.

Determine if the following statements are true or false.

Base angles of an isosceles triangle are congruent.

Base angles of an isosceles triangle are complementary.

Base angles of an isosceles triangle can be equal to the vertex angle.

Base angles of an isosceles triangle are acute.

Fill in the proofs below.

Given
: Isosceles
, with base angles
and
is the angle bisector of
Prove
:
is the perpendicular bisector of

Statement

Reason

1.

1. Given

2.

2. Base Angles Theorem

3.

3.

4.

4. Reflexive PoC

5.

5.

6.

6.

7.

7. CPCTC

8.
and
are supplementary

8.

9.

9. Congruent Supplements Theorem

10.
is the perpendicular bisector of

10.

Given
: Isosceles
with
and
is the perpendicular bisector of
Prove
:
is the angle bisector of

Statement

Reason

1.

1.

2.

2.

3.

3.

4.

4.

5.

5.

6.

6. CPCTC

7.
is the angle bisector of

7.

On the
plane, plot the coordinates and determine if the given three points make a scalene or isosceles triangle.

Vocabulary

Language:

The side of a triangle parallel with the bottom edge of the paper or screen is commonly called the base. The base of an isosceles triangle is the non-congruent side in the triangle.

Base Angles

Base Angles

The base angles of an isosceles triangle are the angles formed by the base and one leg of the triangle.

Base Angles Theorem Converse

Base Angles Theorem Converse

The base angles theorem converse states if two angles in a triangle are congruent, then the sides opposite those angles are also congruent.

Isosceles Triangle Theorem

Isosceles Triangle Theorem

The Isosceles Triangle Theorem states that the perpendicular bisector of the base of an isosceles triangle is also the angle bisector of the vertex angle.

Triangle Sum Theorem

Triangle Sum Theorem

The Triangle Sum Theorem states that the three interior angles of any triangle add up to 180 degrees.

Vertical Angles

Vertical Angles

Vertical angles are a pair of opposite angles created by intersecting lines.

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Description

Apply the properties of isosceles triangles.

Learning Objectives

Here you'll learn the definition of an isosceles triangle as well as two theorems about isosceles triangles: 1) The angle bisector of the vertex is the perpendicular bisector of the base; and 2) The base angles are congruent.